In recent years, treatment of ballast water stored in a ship has become an issue. Ballast water is sea water that is stored in a ship in order to enable the ship to sail safely when the ship is not carrying cargo. Ballast water is taken into a ship from the ocean near a departure port and is discharged to the ocean near a destination port. That is, in a case where sea water taken into a ship as ballast water at a departure port is discharged at a destination port, such as a case where an oil tanker leaves a port in Japan and arrives at a port in a Middle East oil-producing country, such as Kuwait, and loads oil at the port, sea water in a sea area in Japan is taken on board the ship as ballast water and discharged to a sea area in the Middle East. In such a case, because ballast water is discharged to a sea area that is different from a sea area from which the ballast water was originally taken, organisms in the sea water are transferred to a sea area that is not their native habitat and may have a significant effect on marine ecosystems.
Therefore, various methods for purifying ballast water to remove, kill, or inactivate microorganisms have been examined. For example, PTL 1 describes a method for killing aquatic organisms by heating sea water. PTL 2 discloses a method using steam, a method using ultraviolet irradiation, electrical methods using voltage application and an impact wave, a method using a chemical agent such as sodium hypochlorite, and the like. Filtration methods have been also examined as pretreatment before performing killing treatments described above or in order to remove comparatively large microorganisms. For example, PTL 3 discloses a process for producing ballast water by using a membrane filter.
The inventors examined such treatment of ballast water and disclosed a filtration apparatus including a rotating cylindrical pleated filter described in PTL 4. The ballast water treatment apparatus includes a filter that is cylindrically formed so as to surround an axis and that is rotatable around the axis; an untreated water nozzle that ejects untreated water toward an outer peripheral surface of the filter; a case that includes an outer cylindrical portion that is disposed so as to surround the filter, a nozzle opening of the untreated nozzle being located inside the case; a filtered water channel through which filtered water that has passed through the filter flows from a cylindrical space inside the filter to outside of the case; and a discharge channel through which discharged water that has not passed through the filter is discharged to outside of the case. With such a structure, untreated water is ejected toward the outer peripheral surface of the cylindrically formed filter from the outside of the filter, so that untreated water is filtered while the filter rotates and the surface of the filter continuously moves. Thus, it is possible to continue filtering untreated water while preventing the filter from being clogged, and it is possible to remove foreign substances from untreated water more efficiently than in a case where a flat filter having the same area is used.